stubtest_stdlib: suppress output from pip (#9807)

tests/stubtest_stdlib.py

@@ -23,6 +23,7 @@ def run_stubtest(typeshed_dir: Path) -> int:
     combined_allowlist = f"{sys.platform}-py{sys.version_info.major}{sys.version_info.minor}.txt"
     with tempfile.TemporaryDirectory() as tmp:
         venv_dir = Path(tmp)
+        print("Setting up an isolated virtual environment...", file=sys.stderr)
         try:
             pip_exe, python_exe = make_venv(venv_dir)
         except Exception:
@@ -30,10 +31,20 @@ def run_stubtest(typeshed_dir: Path) -> int:
             raise
 
         # Install the same mypy version as in "requirements-tests.txt"
-        subprocess.run([pip_exe, "install", get_mypy_req()], check=True)
+        try:
+            install_command = [pip_exe, "install", get_mypy_req()]
+            subprocess.run(install_command, check=True, text=True, capture_output=True)
+        except subprocess.CalledProcessError as e:
+            print(e.stderr, file=sys.stderr)
+            raise
 
         # Uninstall setuptools from the venv so we can test stdlib's distutils
-        subprocess.run([pip_exe, "uninstall", "-y", "setuptools"], check=True)
+        try:
+            uninstall_command = [pip_exe, "uninstall", "-y", "setuptools"]
+            subprocess.run(uninstall_command, check=True, text=True, capture_output=True)
+        except subprocess.CalledProcessError as e:
+            print(e.stderr, file=sys.stderr)
+            raise
 
         cmd = [
             python_exe,